1. Field of the Invention
The present invention relates to an apparatus and method for control of the shape of the solidification interface of a crystal being grown, and more specifically for controlling the behavior of a peripheral edge of the solidification interface during crystal growth by the Czochralski method.
2. Description of Related Art
The well-known Czochralski method of growing crystals involves contacting a seed crystal with a melt of the crystal material contained in a heated crucible. The growth of large, substantially defect-free crystals by this and other methods remains to this day a difficult tasks, due to complications brought about by temperature gradients and convective eddies within the melt, and continuously changing thermal conditions.
Previous attempts at controlling the crystal growth process to improve the quality of the crystals produced having primarily involved controlling process parameters such as crucible or crystal rotation seeds, crystal pulling rate, and total heat supplied to the crucible application of magnetic fields has also been used in attempting to suppress convective eddies within the melt. A further approach directed to suppressing or eliminating convection cells below a solidification interface has been to provide a baffle or other solid object in the crucible in the vicinity of the solidification front. All of the above approached share the disadvantage that the controls tend to produce only bulk effects within the melt, and generally do not have any substantial effect on controlling the shape and stability of the solidification interface as the crystal is pulled, which is possibly the most critical aspect of crystal growth, in terms of minimizing or substantially eliminating dislocations or other defects formed when growing large single crystals. The shape and stability of the solidification interface also has a substantial effect on whether the material solidifies into a desired monocrystalline structure, or into an undesirable polycrystalline structure.
Optical or electro-optical monitoring of the melt at the melt-crystal interface and controlled cooling of the pulled crystal are other known approaches directed to improving the quality of the crystal end product. Examples of devices and methods for controlled cooling (control of heat flux) of the pulled crystal can be seen in the Motakef patent (U.S. Pat. No. 4,822,449) and the Gevelber patent (U.S. Pat. No. 4,857,278). Each of these patents discloses that it may be possible to also attempt to influence the interface morphology by submerging or penetrating a lower end of the device into the melt. This has the disadvantage that it is an intrusive device and will introduce a further set of complications arising from the presence of a physical structure immersed in the melt and surrounding the interface.
External surface heating of the melt to maintain a predetermined temperature just above the melting temperature of the material has also been proposed, as evidenced in U.S. Pat. Nos. 4,133,969 and 4,650,540. None of these approaches are believed to provide adequate control over the thermal conditions in the melt in the immediate vicinity of the solidification front of the pulled crystal.
The use of segmented heaters, an example of which may be found in U.S. Pat. No. 4,604,262, issued to Nishizawa, has previously been proposed for crystal pulling processes. Such segmented heaters, wherein the segments have different heat outputs, have heretofore been employed to produce an overall vertical temperature gradient in a melt in a crucible, in accordance with a general principle that it is desired to have the temperature of the melt gradually decrease from bottom to top.
One particularly troublesome area in maintaining a stable solidification interface shape is at the peripheral edge region of the solidification interface, where, due to the prevailing thermal conditions and effects of the phase change from liquid to solid, an undesirable "edge downturn" or inflection tends to develop. The edge downturn is undesirable in that it may increase stress levels in the solidified crystal and tends to shift the growth mechanism from that of growing a single crystal to that of formation and growth of a polycrystalline structure.
One approach to specifically controlling the shape of the solidification interface has previously been proposed in U.S. Pat. No. 4,971,652, by the inventor of the present application and assigned to the assignee of the present application. In that application, a heating platform disposed below and extending underneath the solidification interface is provided to monitor and selectively control the temperature gradient across the solidification interface. The positioning of the platform in the melt under the interface makes the use of this invention generally suitable only for processes in which the melt is replenished in the crucible as the crystal is pulled, in order that the solidification interface will remain disposed above the platform.
The ability to predict or model the shape of the solidification interface, coupled with the knowledge that changes in the shape of the solidification interface take place at relatively slow rates due to the relatively low crystal growth rates employed in the Czochralski process, permits reasonably precise control of the process and the shape of the solidification interface without specifically requiring a monitoring system including closed-loop feedback and means for providing instantaneous response to detected changes in various process conditions. The above-noted edge downturn problem is particularly well suited to be controlled without closed-loop feedback, as the problem occurs at the periphery of the interface and can be controlled as set forth hereinafter by selective control of the thermal conditions at a target region at the level of the interface.
It is therefore a principal object of the present invention to provide a crystal growing apparatus having means for maintaining a stable solidification interface shape at a peripheral edge region of a crystal being grown.
It is another important object of the present invention to provide a method for controlling and maintaining a desired shape of a peripheral edge region of a solidification interface of a crystal being grown.
It is another important object of the present invention to provide a non-intrusive, individually controllable heater, separate from the bulk heater, to effect a desired temperature balance at the edge region of solidification interface to maintain a desired shape of the peripheral edge region.
It is a further important object of the present invention to provide an apparatus and a method employing the apparatus which will yield single crystals grown by the Czochralski method having improved size, yield, and quality, particularly with respect to the production of a uniform, substantially dislocation-free, monocrystalline structure.